With the current evolution in the standards on exhaust emissions from vehicles, devices for pollution control of NOx (or nitrogen oxides) have had to be put in place, especially on trucks (heavy goods vehicles).
The system used by most vehicle manufacturers for reducing NOx emissions to the required value generally consists in carrying out a selective catalytic reaction with reducing agents such as urea (“Urea SCR” or selective catalytic reduction using ammonia generated in situ in the exhaust gases by decomposition of urea).
In order to do this, it was necessary to equip the vehicles with a tank containing a urea solution, a device for metering the amount of urea to be injected into the exhaust line and a device for supplying the urea solution from the tank to the injector. Generally, this supply device comprises a pump.
As it is the case with other fluid injection systems on vehicles, like fuel injection for instance, it is preferable to protect the pump against particles that may be contained in the additive leaving the tank, and to protect the metering device (if metering is not performed directly by the pump and/or injector) and the injector from contamination by wear particles produced by the pump.
The first problem is classically solved by putting a filter in between the tank and the pump. As to the second one, U.S. Pat. No. 6,125,826 proposes to solve it in the case of a fuel injection system, by inserting a self-washing filter of given geometry between the pump and the metering device. This filter provides a filtered fuel flow to a metering device and a particle containing fuel flow which is returned through a bypass valve to a position upstream of the main filter. Such a system only works with injection systems involving a return line and where the fluid to be injected always circulates in the same direction.
However, the aqueous urea solutions generally used in SCR (eutectic water/urea solutions with 32.5% by weight of urea) freeze at −11° C. so it is necessary to provide not only a heating device (to liquefy the solution in order to be able to inject in freezing conditions) but also: a purge system in order to be able to periodically purge the lines that convey the urea in order to clear them of the liquid deposits of additives, which are liable to solidify in case of frost.
An effective way to perform such a purge is described in patent application WO 2006/064028 in the name of the Applicant. It uses a purge device suitable for forcing a purge gas to flow through the entire injection line from the injector to the tank, which consists in:                either a compressed air tank and a purge line equipped with a valve for passing the compressed air through the said line;        or a switch for reversing the flow of the pump (6);        or a 4/2-way valve (14) which, using suitable fittings/lines, produces the same effect.        
The two last alternatives are preferred because they do not involve an additional compressed gas tank. The purge gas of these embodiments consists in fact of engine exhaust gas and/or ambient air which is sucked into the injection line by a suitable device, via the exhaust line. If the pump can operate in two opposing directions and generate reverse flows, this device may be a simple switch that has the effect of reversing the flow of the pump. With standard pumps, the purge device may be a 4/2-way valve, which, in normal mode (additive injection) connects a point downstream of the pump to the injector, and in purge mode, connects the injector to a point upstream of the pump—which has somewhat the same effect as reversing the flow of the pump (the upstream and downstream points being reversed by the valve).
When the system of WO'028 is working in purge mode, particles that were trapped on the secondary filter (i.e., the one that is generally present on the injector (integrated to it) and/or the additional one as described in U.S. Pat. No. '826 mentioned above) are pulled back into the pump and trapped on the primary filter. Then, when the pump is put in feed mode again, these particles are pulled back into the pump again until they reach the secondary filter etc . . . causing additional wear and damage to the pump.